Polarization mode dispersion (PMD) is a physical effect caused by the inherent birefringence of optical materials. In optical fibers, this inherent birefringence is induced by several factors, including manufacturing imperfections and stresses, microbends, and the like. As a result of the PMD associated with the optical fibers and other components of an optical communications network, the transmission data rate for a given link length is limited. Projects aimed at controlling the PMD associated with optical links are being undertaken at laboratories around the world. For example, polarization mode dispersion compensators (PMDCs) and the like are being developed.
In order to compensate for the effects of PMD, the polarization behavior of the signal must be dynamically tracked as a function of the environmental conditions of the various components that make up an optical link. The dynamic aspects present impose stringent time response requirements onto the control hardware and algorithms. DCMs are one critical component that can induce rapid polarization changes. These modules are passive in nature and are made of a special type of optical fiber, namely dispersion compensation fiber (DCF). Typically, DCF is packaged inside a DCM with little or no attention to the resulting polarization changes that can occur during a shock or vibration. All conventional optical fiber DCMs suffer from this lack of polarization control.
There are many sources of polarization changes in an optical fiber transmission system. These polarization changes can be on the order of ˜0.2π/sec to ˜10,000π/sec, depending upon the shake, bang, etc. that is encountered. The polarization control hardware and algorithms utilized must be specified to deal with the fastest potential transient encountered, and DCM induced transients have been shown to be at least 10× faster than the next fastest transients. Thus, eliminating polarization changes due to DCMs would allow a corresponding 10× reduction in the required control hardware and algorithm demands. This reduction would translate into reduced cost, reduced power consumption, etc.